Usually, the speed of a vehicle is derived from the rotational speed of a wheel thereof. However, due to the slippage of the wheels such speed measurement is rather inexact, in particular during braking of the vehicle. Now, automatic braking systems have been introduced in the recent years. These systems are based on the behaviour of the wheels which might be quite different from that of the movement of the vehicle when there is a remarkable slippage.
The German Laid Open Patent Publication No. 34 35 866 discloses a method for determining the actual speed of a vehicle. In order to avoid any influence of the slippage between the wheels of the vehicle and the road it is proposed to pick-up the oscillations of the wheels caused by the road unevenness and to determine the actual absolute speed of the vehicle by evaluating the time difference of the oscillations of the front and rear wheels.
The German Laid Open Patent Publication No. 27 51 012 discloses an apparatus for determining the speed of a vehicle by means of two transducers arranged in a fixed distance in the direction of movement of the vehicle. The transducers generate electrical signals reflecting unevennesses of the road surface. The signals of the two transducers are correlated with each other for determining the time delay therebetween which is multiplied by the fixed distance in order to determine the actual speed. The vehicle is further provided with a measuring device for determining the distance covered by the vehicle. This device is operated by the rotation of the wheel. The reading of the distance measuring device is corrected by the integrated signal of the determined actual speed.
The German Laid Open Patent Publication No. 28 49 028 discloses a similar device for determining the actual speed of a vehicle by cross-correlating signals derived from sensors arranged at a pre-determined distance in the direction of movement of the vehicle. Specifically, mechanical-to-electrical transducers are used formed by strain gauges, force or acceleration pick-up devices provided at the wheel suspensions, the springs or shockabsorbers or bumpers of the vehicle.